Phillip's Natural World: Flash Drought

A massive upper-level high will envelop most of the lower 48 in the last half of July, setting up what could be a prolonged bout of extreme heat for millions of Americans. If the scorching weather persists into August, the odds of a flash drought in the nation's heartland will rise sharply (along with the odds that the United States will have its hottest summer on record along with what is very likely going to be Earth's hottest year on record).

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There's going to be no escaping the misery of the pattern projected for the next few days at upper levels, the upper high at 500 millibars (about four miles up) encompasses nearly all of the lower 48 by Thursday night, July 21. Because air expands as it warms, a higher 500-millibar surface is associated with a warmer air mass at lower levels. Computer weather forecast models agree on development of a mammoth upper ridge centered near the nation's Midwest for the next week to 10 days, perhaps longer. The atmospheric variables predicted to take shape later this week are similar to those observed during some of the nation's most infamous heat waves of recent decades.

The closest analog to the pattern developing is July 13, 1995, the second day of a catastrophic 5-day heat wave that took more than 700 lives in the Chicago area. The top analogs also include July 4 2012, and August 27, 1980, two peak days from the devastating central US drought years of 2012 and 1980. Excessive heat watches for later this week have already been issued by the National Weather Service for parts of Illinois, Iowa, Kansas, Minnesota, Nebraska, North Dakota, and South Dakota. We can expect many other areas to follow suit as the week unfolds.

Dangerous Heat

Scorching Temperatures and Wilting Humidity

With such strong computer forecast model support for a high-end upper ridge (heat wave), one might expect surface temperatures to be corresponding extreme. Indeed, readings near of above 100° (38° C) are likely to encompass large parts of the Great Plains by midweek, perhaps even topping 110° (43° C) in some spots with readings of 90°-100° over a large part of the nation. As an upper-level impulse rides along the north side of the ridge later this week, a burst of heat and humidity will be shunted eastward, approaching the East Coast toward the weekend. Highs are forecast to be in the 105° range this weekend in Washington, DC, approaching the city's all-time high of 106° set on August 6, 1918, and July 20, 1930.

Another important element of this heat wave: Some of the energy that would otherwise go into heating up the lower atmosphere will be diverted into evaporating moisture. Plants and soils are quite moist in many areas thanks to recent rains, especially through a belt from the Central Plains into the Ohio Valley. That moisture is a mixed blessing: While it will help to keep surface air temperatures a notch lower than they would be otherwise, it will also help raise the amount of water vapor in the air. In addition, the Midwest's vast corn crops are at a stage where they add moisture to the air through evapotranspiration, a process dubbed "corn sweat." As a result, heat index values will soar to uncomfortable and even dangerous levels as this week progresses over large parts of the central and eastern USA, especially toward the south. The atmospheric moisture will also help boost nighttime lows, which exacerbates the potential risk to human and animals health from a multi-day heat wave.

EDDI Flash Drought Prediction

New research has led to the emergence of new monitoring efforts that may help identify and even predict flash droughts. The National Drought Mitigation Center has been leading a multiyear effort to develop the Quick Drought Response Index (QuickDRI), which will monitor changes in vegetation over periods of a week or two. QuickDRI is building on VegDRI, an operational product that uses satellite and climate data to map vegetation change at the seasonal scale. Funded by NASA, QuickDRI is being tested this year.

Meanwhile, a group led by Michael Hobbins (NOAA/ESRL Physical Science Division) and Dan McEvoy and Justin Huntington (Desert Research Institute) has developed the Evaporative Demand Drought Index (EDDI), which focuses on the weather that drives both flash drought and long-term drought. Rather than assessing the landscape itself, or recent rainfall, EDDI looks solely at evaporative demand—the impact of atmospheric temperature, humidity, wind, and solar radiation over a particular time period—and how it compares to climatology. A positive EDDI indicates drier-than-average conditions. In evaluations thus far, EDDI appears to work well in providing advance notice of drought development, often ahead of other commonly used indexes. The EDDI is spotlighted in the June issue of the Journal of Hydrometeorology, where a pair of papers explains the rationale for the index and a U.S.-based evaluation of its skill.

Regularly updated EDDI maps for various time frames are available for download. The most recent 4-week EDDI map suggests that the eastern Great Lakes, New York, and western New England have experienced drought-favorable weather over the last month. Patches of moderate drought in these areas are now showing up in the weekly U.S. Drought Monitor. Central Florida is also exhibiting drought-favorable conditions.

Flash Drought

In 2012 a promising-looking spring morphed into a terrible summer for the US Midwest. A long-term drought that began in late 2010 had intensified over the Southern Plains in 2011, punishing farmers and ranchers and facilitating the loss of roughly 10% of all the trees in Texas. The real shock was how quickly drought conditions took hold further across the Midwest in the summer of 2012, leading to the most widespread US drought conditions since the 1930s. Even NOAA's 30-day and season drought outlooks from June 2012 failed to predict that month's emergence of drought in the Midwest,. It appears this wasn't a simple northward extension of the ongoing drought further south, but something else, a classic case of what's increasingly known as flash drought, a rapid-onset drying of the landscape.

While long-term drought can emerge simply through a lack of precipitation, a flash drought is closely linked to hot summer weather. The type of flash drought most often observed in the Midwest develops as a torrid air mass sweeps in for a period of a few days to several weeks. At first, the landscape may not be particularly dry, in which case large amounts of water vapor flow from vegetation and soils into the scorching surface air (as is expected later this week). If the heat is strong and sustained enough, the landscape quickly dries and flash drought takes hold.

Photo: Cristobal Herrera/European Pressphoto Agency

Miles of Algae
and a Multitude of Hazards

The stench from decaying algae began rising from coastal waterways in southeastern Florida early this month, shutting down businesses and beaches during a critical tourism season. Officials arrived, surveyed the toxic muck and declared states of emergency in four counties, then took off for Cleveland for the Republican National Convention.

Photo: Rhonda Wise/Agence France-Press/Getty Images

Photo: Greg Lovett/The Palm Beach Post/AP

Florida Governor Rick Scott doesn't believe in global warming or climate change anymore than does Donald Trump or Florida Senator Marco Rubio, and why should they care? They don't live amongst the dead fish and dying waterways of southeast Florida, and their reelection campaigns are largely funded by Big Sugar which polluted Lake Okeechobee and caused the algae bloom in the first place.

Road to Rio